Startup.cs

using IdentityManager2.AspNetIdentity;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;

namespace Host
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<IdentityDbContext>(opt => opt.UseInMemoryDatabase("test"));

            services.AddIdentity<IdentityUser, IdentityRole>(options =>
            {
                options.Password.RequireDigit = false;
                options.Password.RequireLowercase = false;
                options.Password.RequireNonAlphanumeric = false;
                options.Password.RequireUppercase = false;
                options.Password.RequiredLength = 4;
            })
                .AddEntityFrameworkStores<IdentityDbContext>()
                .AddDefaultTokenProviders();

            services.AddIdentityManager()
                .AddIdentityMangerService<AspNetCoreIdentityManagerService<IdentityUser, string, IdentityRole, string>>();
        }

        public void Configure(IApplicationBuilder app)
        {
            app.UseDeveloperExceptionPage();

            app.UseRouting();

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseIdentityManager();

            app.UseEndpoints(x => x.MapDefaultControllerRoute());
        }
    }
}